Combinando consultas mysql que reciclan múltiples uniones automáticas

SELECT  COUNT(*)
FROM    (
        SELECT  session_id
        FROM    formation_page_hits
        WHERE   progress IN (2, 4, 7)
                AND datetime >= '2011-03-23'
                AND datetime < '2011-03-24'
        GROUP BY
                session_id
        HAVING  COUNT(DISTINCT progress) = 3
        ) q

Cree un índice compuesto en (session_id, datetime, progress) para que esto funcione rápido.